Over the past two decades, energy storage technologies based on lithium-ion batteries have proven successful and found widespread use in applications such as portable electronics and consumer devices. Sulfur is a promising cathode material with a high theoretical capacity of 1673 mA h/g based on the electrochemical reaction: S8+16Li⇄8Li2S. An alternative cathode material is fully-lithiated sulfur, Li2S, with a theoretical capacity or 1166 mA h/g. Because the cathode provides a source of lithium, it can be paired with high-capacity lithium metal-free anodes (such as silicon or tin), which avoids dendrite formation and safety concerns associated with metallic lithium anodes.
The polysulfide shuttling reaction between sulfur and its lithiated compounds has limited the development of batteries based on the Li—S chemistry because the reaction leads to irreversible material losses in the battery that reduces energy storage capacity over time. Shuttling is a cyclic process in which long-chain lithium polysulfides, (Li2Sn, 2<n<8), generated at the cathode during charging, dissolve into the electrolyte and migrate to the anode by diffusion where they react with the lithium electrode in a parasitic fashion to generate lower-order polysulfides, which diffuse back to the sulfur cathode and regenerate the higher forms of polysulfide. Since this polysulfide shuttling or dissolution takes place at the expense of the available electroactive sulfur species, the reversibility of sulfur and/or sulfide is broadly considered a major technical barrier towards commercialization of high-energy Li—S batteries.
Additives like metal oxides (SiO2, TiO2, Al2O3 etc.) and binders that possess strong Li—O interaction such as poly(vinylpyrrolidine) have been used in cathodes to address battery stability. However, the addition of metal oxides non-electroactive metal oxide add to the volume of the electrode and can reduce energy density. In addition, due to its insulating nature, metal oxide additives can be detrimental to high rate performance.
Another limitation is elemental sulfur is a poor electrical conductor (with a Conductivity≈5×10−30 S cm−1 at 25° C.), which has limited the rate at which a conventional Li—S battery can be discharged/charged. To address the inherent low conductivity of sulfur, electronically conductive additives, such as conductive carbon, can be added to the cathode composition.
There remains a need for sulfur-containing cathode materials for lithium secondary cell with improved conductivity and cycle life.